The present invention relates to fungi having dioxin degradability, to a method of using the fungi for degrading dioxins, to compost that contains the fungi, to a method for producing the compost, and to a method of using the compost for cultivating plants. More precisely, the invention relates to fungi capable of degrading dioxins having accumulated in the soil of farms and developed lands into harmless substances, to a method of using the fungi for degrading dioxins, to compost that contains the fungi, to an effective method for producing the compost, and to a method of using the compost for cultivating plants for agricultural crops, etc.
Dioxins are a type of harmful substances that are discharged in the nature from various facilities, appliances and others typically including incinerators for garbage and industrial wastes.
Known are various dioxins of different chemical structures, and 2,3,7,8-tetrachlorodibenzo-p-dioxin is the most typical. Living organisms could hardly degrade dioxins, and many of them absorbs dioxins. Through food chain, dioxins are finally accumulated and concentrated in animals, often causing malformation in the animal kingdom, and the problem with dioxins is now a grave object of public concern.
Various methods for preventing the generation of dioxins have been investigated and proposed. For example, a two-stage method of burning exhaust gas at high temperatures in automobiles and incinerators has been put into practical use. However, this is still insufficient for preventing the generation of dioxins. Dioxins having been discharged in air fall down on the earth along with rain and snow, and accumulate in soil. At present, no one knows an effective method for degrading dioxins into harmless substances.
Some recent studies are directed to biodegradation of chemical substances such as dioxins and others that are hardly degraded in the nature, and some reports say that lignin degrading enzymes produced by a type of microorganisms can degrade dioxins [BIO INDUSTRY, Vol. 15, No. 2, pp. 5-13 (1998); Chemistry, Vol. 52, No. 10, pp. 24-25 (1997)].
Relative to the degradation of dioxins by such microorganisms-derived lignin degrading enzymes, the reports precisely say that the lignin degrading enzymes produced by white rot fungi, which are in a group of wood-rotting fungi of the genus Basidiomycetes, have the ability to degrade various chemical substances including dioxins, further saying that the white rot fungi grow with a nutrient source of polysaccharides, cellulose and hemicellulose that are the essential ingredients of wood, and degrade lignin in wood depending on the energy from the nutrient source. Accordingly, in wooded regions where such white rot fungi live, dioxins having fallen down on the earth along with rain shall be degraded by lignin degrading enzymes produced by the white rot fungi.
In farms for agricultural crops, grass, etc., however, the soil is poor in cellulose and hemicellulose that are to be the nutrient source for white rot fungi. In addition, a large amount of microbicides and chemical fertilizers are often applied to the soil of such farms. Therefore, the environment of farms is not suitable for white rot fungi living therein. In some area of lands developed by civil engineering, trees and flowers are planted. However, the soil in the area is also poor in cellulose and hemicellulose, and its environment is not suitable for white rot fungi living therein.
Accordingly, dioxins will accumulate increasingly in such farms and developed lands, and the influence of the thus-accumulated dioxins on living organisms will be a more serious problem. In that situation, it is much desired to develop a technique effective for degrading the dioxins having accumulated in the soil of farms and developed lands into harmless substances to thereby renew the soil so that the renewed soil may have no negative influence on living organisms.
The present invention is to provide fungi having the ability to degrade dioxins that have accumulated in the soil where agricultural crops and others are cultivated into harmless substances, to provide compost that contains the fungi and a method of producing the compost, and to provide a method of using the compost for cultivating plants.
We, the present inventors have assiduously studied to solve the problems noted above, and, as a result, have found that, when a plant-derived organic material that contains lignin is used for producing compost and when the plant-derived organic material having been thermally fermented under a specific condition is inoculated with a specific type of fungi to be compost, then the resulting compost can degrade dioxins having accumulated in the soil of farms and others into harmless substances. On the basis of these findings, we have completed the present invention.
Specifically, the subject matter of the invention resides in the following:
1. Fungi having the ability to degrade lignin and belonging to the genera, Chaetomium, Penicillium and Schizophyllum, which have the ability to degrade dioxins.
2. A method of degrading dioxins, which comprises applying the fungi of above 1 to a dioxin-containing organic or inorganic substance in an amount of at least 1xc3x97102 cfu per gram of the dry weight of the substance.
3. A method of degrading dioxins by applying the fungi of above 1 to a dioxin-containing organic or inorganic substance, for which is used a material inoculated with the fungi.
4. Compost containing the fungi of above 1.
5. The method of degrading dioxins as in above 3, wherein the fungi-inoculated material is the compost of above 4.
6. A method for producing the compost of above 4 by fermenting a plant-derived organic material, which is characterized in that the plant-derived organic material contains lignin, and the plant-derived organic material having been thermally fermented in a process that includes a step of keeping it at a temperature falling between 65 and 100xc2x0 C. for at least 2 hours is further fermented while being inoculated with at least one type of the fungi of above 1 at a temperature not higher than 65xc2x0 C.
7. A method for producing the compost of above 4, wherein the plant-derived organic material is inoculated with the fungi of above 1 to such that the cell concentration in the inoculated material is at least 1xc3x97102 cfu per gram of the dry weight of the plant-derived organic material.
8. A method for producing the compost of above 4, wherein the plant-derived organic material is thermally fermented in the presence of a nitrogen fertilizer component-containing substance that serves as a fermentation promoter, in the initial stage of the process of thermophilic fermentation.
9. Compost having the ability to degrade dioxins, in which the cell concentration of the fungi of above 1 falls between 1xc3x97103 and 1xc3x971010 cfu per gram of the dry weight of the compost.
10. A method for cultivating plants, which comprises applying the compost of above 4 to the soil of farms or developed lands, and cultivating plants on the soil.
In the method of the invention for producing compost that has the ability to degrade dioxins, used is a lignin-containing, plant-derived organic material for the compost.
In general, weeds and vegetable refuse that can be fermented into compost within a short period of time are much used for the material for compost that serves as a soil improver or base manure. However, as not containing a large amount of lignin, they are unsuitable for the material for the compost of the invention. Even if used, they could not attain the intended effect of the invention. A lignin-containing, plant-derived organic material is suitable for the compost of the invention. Specific examples of the material are wood, bark, bamboo, corn stems, pulp, pulp wastes, etc. For their forms, wood chips and sawdust are preferred. Also preferred are bark, bamboo and corn stems having been cut into pieces to have a length of 200 mm or so; as well as their milled powders, pulverized chips or chopped pieces.
For maturing it into compost, the compost material as above is fermented in any ordinary compost fermenter. In general, in a process of producing compost from such a compost material, microbes intrinsic in the compost material, those from the nature, or those intrinsic in additives first act on the material to ferment it, then the material is thermally fermented by thermophilic anaerobic bacteria existing therein, and after the peak stage of thermophilic fermentation, the material having been thus thermally fermented is gradually to be in the latter stage of fermentation where it receives no more heat, and finally the step of fermenting the material is completely terminated. Through the process, the compost material is matured into compost.
Also in the invention, a lignin-containing, plant-derived organic material is fermented in the same manner as above. As compared with ordinary compost materials, the lignin-containing, plant-derived organic material for use in the invention is relatively slowly matured into compost since its nitrogen content is small. Therefore, in the initial stage of fermentation, it is desirable to add thereto a fermentation promoter of, for example, animal feces, activated sludge wastes, stock farm wastes and fisheries wastes, or synthetic fertilizer such as ammonium sulfate, urea and the like that serve as a nitrogen source. With such additives being added thereto, thermophilic fermentation of the compost material can be promoted in its initial stage. The amount of the fermentation promoter to be added to the plant-derived organic material falls, for example, between 5 and 30 kg of the dry weight of animal feces per ton of the dry weight of the organic material; and the amount of the synthetic fertilizer to be added thereto falls, for example, between 5 and 30 kg per the same. In case where both the two, fermentation promoter and synthetic fertilizer are used together, the total weight of the two preferably falls between 5 and 30 kg. The fermentation promoter is preferably fed into a fermenter along with the starting, plant-derived organic material thereinto. For example, it is desirable that the compost material and the fermentation promoter are alternately piled up in a compost fermenter so that the promoter can be uniformly dispersed in the compost material in the fermenter.
During the process of thermal fermentation, the water content of the starting, plant-derived organic material to be compost is controlled to fall between 40 and 85%, preferably between 45 and 70%. In order that the compost material can undergo uniform thermal fermentation, it is desirable that the pile of the compost material in the fermenter is aerated by turning it inside out twice to ten times during the process of thermal fermentation. Though varying depending on the type of the compost material to be fermented, the type of the additive thereto and their blend ratio, the period of thermal fermentation is generally a few weeks or longer.
The process of thermal fermentation necessarily includes a step of keeping the compost material at a temperature falling between 65 and 100xc2x0 C. for at least 2 hours. The reason why the compost material being fermented shall be kept at the temperature falling within that range for at least 2 hours is for preventing undesirable microbes having contaminated the compost material and the additive from growing or for killing them so that they have no more negative influences on the next step where the compost material is inoculated with fungi and the thus-inoculated fungi grow in the material.
Next, the plant-derived organic material having undergone the thermal fermentation or the thus-fermented compost material is inoculated with fungi having the ability to degrade lignin, at a temperature not higher than 65xc2x0 C., and is further fermented with the fungi. The temperature at which the compost material is inoculated with such fungi shall not be higher than 65xc2x0 C. This is because, at temperatures higher than it, the fungi will die. Preferably, the temperature is not higher than 50xc2x0 C.
Especially preferably, the fungi having the ability to degrade lignin for use in the invention are at least one type of fungi selected from a group of fungi of the genera, Chaetomium, Penicillium and Schizophyllum. Specific examples of these fungi are Chaetomium virescens, Chaetomium reflexum, Chaetomium piluliferum, Chaetomium thermophilum, Chaetomium gracile, Chaetomium indicum, Chaetomium globosum and Chaetomium dolichotrichum that are fungi of the genus Chaetomium; Penicillium janthinellum, Penicillium funicolosum, Penicillium asperosporum, Penicillium crustosum and Penicillium verruculosum that are fungi of the genus Penicillium; and Schizophyllum commune of fungi of the genus Schizophyllum.
For cultivating the culture on which the fungi are inoculated, employable is any method of cultivating ordinary fungi. For example, on a laboratory scale, the fungi are cultivated in a potato-dextrose medium at 25xc2x0 C. for 10 days. For mass-production on an industrial scale, employable is any of a method of liquid culture in a tank, and a method of solid culture using, for example, a plant-derived solid component such as whole wheat grains, or an inorganic porous carrier with sugar, nitrogen, phosphorus, minerals and the like being infiltrated thereinto. In order to make the fungi have increased dioxin degradability, it is desirable that a lignin material is added to the medium of solid culture where the fungi are cultivated. In the process of cultivating the fungi, the resulting culture is so controlled that the cell concentration therein is at least 1xc3x97102 cfu (colony formation unit), preferably from 1xc3x97102 to 1xc3x97108 cfu, more preferably from 1xc3x97103 to 1xc3x97107 cfu, per gram of the dry weight of the plant-derived organic material. The reason why the cell concentration is controlled to fall within the defined range is because, if it is smaller than the range, the growth of the fungi inoculated will be retarded. If so, in addition, the fungi inoculated will be difficult to grow predominantly over any other existing microbes.
The fungi to be cultivated may be any of mycelia or spores. In view of their storage stability, however, it is desirable that a culture that contains cells having well grown to have a lot of spores is first prepared, and thereafter this is inoculated on a plant-derived organic material. For this, for example, in case where the fungi are cultivated according to liquid culture, supplying a carbon source to the culture medium is stopped just before the end of culture, or the pH value of the culture medium is rapidly varied, or the amount of aeration into the culture medium is reduced, whereby the sporulation is promoted. The culture of the fungi thus obtained is preferably used in the invention.
The culture of the fungi having been thus cultivated may be directly applied to the compost material to be inoculated therewith. If desired, however, the culture may be ground or cut into pieces, or may be centrifuged or sieved to separate the cells or the spores or their mixtures from the culture, and the thus-processed culture may be applied to the compost material. Also if desired, the culture may be dried at a temperature at which the cells do not die, for example, at a temperature not higher than 50xc2x0 C., and the dried culture may be applied to the compost material. For drying the culture, employable is an aeration drying method, or a freeze drying method, or a method of mixing it with a dry carrier (for example, bran, inorganic carrier, etc.) to lower its water content.
The culture of the fungi thus prepared is inoculated on a plant-derived organic material at a temperature not higher than 65xc2x0 C., preferably not higher than 50xc2x0 C. This is because, if the temperature of the plant-derived organic material is higher than 65xc2x0 C., the fungi inoculated on the material will die. For inoculating the thus-cultivated fungi on the plant-derived organic material, the cell culture may be directly applied to the material. Apart from the direct inoculation method, the cell culture is inoculated on a plant-derived organic material having been thermally fermented or on a plant-derived solid component to thereby further grow the cells, and the culture with the thus-grown cells therein may be inoculated on the compost material. As the case may be, the compost produced according to the method of the invention may be used for cell inoculation.
The plant-derived organic material thus inoculated with the fungi is fermented at a temperature falling between 10 and 50xc2x0 C. This fermentation may be effected in the same field where the compost material has been thermally fermented in the previous stage. Alternatively, the material inoculated with the fungi may be embedded in soil or may be piled up for fermenting it. In this fermentation stage, the fungi inoculated on the plant-derived organic material grow. The preferred temperature in this stage varies, depending on the type of the fungi fermented on the material. At temperatures lower than 10xc2x0 C., the growing activity of the cells will be low; but at temperatures higher than 50xc2x0 C., the cells except some types of fungi will die.
Preferably, the plant-derived organic material contains water to some degree. In order that the fungi inoculated thereon could be active, the water content of the plant-derived organic material preferably falls between 40 and 85% by weight, more preferably between 45 and 70% by weight. If the water content of the material is lower than 40% by weight, the growth of the fungi in the material will be retarded; but if higher than 85% by weight, the material being fermented will be readily in an anaerobic condition. In such an anaerobic condition, the inoculated fungi will die. In case where the water content of the material is high and therefore the material will be in an anaerobic condition, it is desirable that the material being fermented is aerated by turning it inside out to some degree, or by forcedly introducing air into the material so that the material could be at least in a slightly aerobic condition.
Preferably, the pH value of the plant-derived organic material being fermented falls between 4 and 9. If the pH value of the material is smaller than 4 or higher than 9, the growth of the fungi in the material will be retarded. In case where the pH value of the material being fermented oversteps the range of from 4 to 9, it is desirable that an acidic or alkaline substance such as superphosphate, calcium oxide, slaked lime or the lime is added to the material so as to control the pH value of the material, thereby creating the environment favorable to the growth of the fungi.
The compost of the invention thus produced in the manner as above contains the fungi, having a cell concentration of from 103 to 1010 cfu per gram of its dry weight, and contains manure ingredients highly helpful to the growth of plants. Therefore, it is effectively utilized for good growth of plants by applying it to the soil of farms and developed lands. In the compost of the invention thus applied to the soil, the fungi act to release lignin from the ingredients of the compost, and the thus-released lignin induces an enzyme, fungi-derived lignin degrading enzymes. Accordingly, the compost shall contain the lignin degrading enzymes. The lignin degrading enzymes degrades dioxins in soil into harmless substances.
Where the compost of the invention is applied to the soil of farms or developed lands in which agricultural crops are cultivated or trees or flowers are planted, its amount may be the same as that of ordinary compost. The compost of the invention may be combined with any ordinary compost. Preferably, it may be combined with chemical fertilizer poorly toxic to fungi.
The type of agricultural crops suitable to the compost of the invention is not specifically defined. The compost may be applied to any type of ordinary farm crops including, for example, agricultural and horticultural crops such as vegetables, cereal plants, bean plants, fruit trees, etc., as well as crops for stockbreeding feed, such as grass, etc. The farm crops and feed crops cultivated by the use of the compost of the invention are free from the danger of dioxin intake thereinto, and their harvests are highly safe.
The compost of the invention is useful for tree-planting projects in land developing or civil engineering. For example, it may be blended with the soil or base for cultivating trees, plants and flowers to be planted in the area of developed lands. The trees, plants and flowers for such green zones include, for example, Alnus sieboldiana, Alnus pendula, Alnus Japonica, bush clovers, cericeas, Amorpha fruticosa, white birches, indigos, Japanese privets, Rhaphiolepis umbellata, mugworts, Japanese knotweeds, eulalias, tall fescues, perennial ryegrass, creeping red fescues, etc. The compost of the invention may be applied to the soil where such trees and others are cultivated and grown, and it acts as manure for them and degrades dioxins in the soil into harmless substances.
In addition, the compost of the invention is not limited to its application to soil where plants are cultivated, like that of ordinary compost, but may also be sprayed on the surface of the ground around garbage incineration facilities which will be much contaminated with dioxins. In the contaminated ground on which the compost has been sprayed, the fungi existing in the compost act to degrade dioxins into harmless substances.